1. Field of the Invention
The present invention relates to the reception of digital television signals, and more particularly to the reception of digital television signals in the context of a decoding system using common interface modules operating in a standardized protocol.
2. Description of Related Art
The development of signal processing in digital form in the field of television, as well as the progress of encoding techniques for pay channels, has led to the notion of a common interface (CI) that is meant to be standardized and used on a continental scale.
In this context, it is planned to allow television viewers (“users”) to dispose of a large range of pay television channels (or radio stations) provided by different operators. The digital video signals of the pay channels are transmitted in encoded form, and they are decoded upon reception by a demodulator. For this purpose, the demodulator is piloted by an interface unit that allows the selective decoding of a channel, a group of channels, or an occasional program by codes managed by a chip card or the like personalized to the user as part of a subscription.
The operational elements involved on the reception side to manage the digital data flows between an input of digital signals, which may be encoded, and an audio/video output to a television are represented in FIG. 1. This assembly 2 of elements is divided into two main groups: a host 4 that processes the signals from the reception input to the output to the television, apart from the specific aspects of decoding; and at least one module known as a “common interface” module 6 that decodes the signals.
The host 4 comprises a tuner 6 which receives an input of modulated digital signals (RF signals) 10, emitted for example from a satellite dish or a cable terminal, and which selectively extracts the signals of a channel to transmit them to a demodulator 12. The demodulator extracts from the modulated signal of the selected channel the digital audio and video flows. When this data is encoded, which is the case for pay channels, it must be decoded before it can be used. For this purpose, the digital data is transmitted in the form of flows called “encoded transport” to the input of the decoding unit 16 of the common interface 6, where it is decoded and then sent as an input to the demultiplexer 18 of the host 4.
In this example, the output data from the demultiplexer is in compressed form, using the MPEG standard, and is therefore decompressed by an MPEG decoder 20 which produces a 22V video component output in the form of red, green and blue (RGB) signals and 22 A audio to the corresponding television inputs to reproduce the image and sound.
The host 4 and the interface module 6 are each controlled by their own microprocessor (respectively 24 and 26), with the microprocessor of the host responding to commands sent by the user via a remote control unit 28. Furthermore, the two microprocessors 24 and 26 communicate with one another via a command connection 30. The host 4 is thus connected to the interface module 6 by two types of connections on the respective buses: the command bus 30, which handles the internal control of the assembly; and the transport bus 32, which handles the transmission and reception of the data before and after decoding.
The buses 30 and 32 are controlled by a common interface 34, which comprises a software module that permits the exchanges between the host 4 and each interface module 6 to be standardized.
The personalized data concerning the decoding is contained in a computer memory board or a chip card 36 that can be inserted into a card reader provided for this purpose in the interface module 6.
The interface module 6 is used according to a protocol which specifies the hardware aspects, such as the type of connector (generally in the PCMCIA or PC Card format) that connects it to the host 4 and the flows that the two buses 30 and 32 have to handle, and the software aspects associated with the control of this type of module, such as the dialogue mode between the host and interface module, the connection and disconnection procedures, and so on.
For information, the standard stipulates that when there is provided the possibility of connecting more than one interface module 6 to the host 4, the transport connection interface (bus 32) must be connected in series successively through each of the modules, as shown in FIG. 2.
In this representation, the host 4 is connected to a number n of common interface modules 6, designated by IC1-ICn, each with its own two way transport connection 32. The “return” connection (which transports the decoded data) of a module is also connected to the outward bound connection (carrying the encoded data) of the module next to it, by an internal 32c connection which performs the concatenation between these two modules. This concatenation is repeated in the same way for each pair of adjacent modules in order to connect all of the modules sequentially, with the “return” connection of the preceding module connected to the “outward bound” connection of the following module. A bypass function is provided for each module, so that one or each module that is missing or not used can be bypassed in order to access another.
The host 4 must furthermore maintain the command interface connections of each common interface module 6 simultaneously separate and accessible, so that the transactions between the host and the module can be processed independently for each module. When a common interface module 6 is removed, the transport layer of the command interface connecting any other module must not be disrupted or interrupted.
When several interface modules 6 are connected to a host 4, the host must be able to select the module or each module required to decode the service or each service selected.
In compliance with the standard, the transport layer of the command interface operates above the “link” layer provided by the chosen physical application.
The transport protocol is of the command-response type, where the host 4 sends a command to the interface module 6 via a command transport protocol data unit (C_TPDU) and waits for a reply via a response transport protocol data unit (R_TPDU). The module cannot initiate communications; it has to wait for the host to invite it, or first send it data.
Data communication via the interface unit is defined in terms of objects. The objects are coded using Tag Length Value coding, derived from the coding used for ASN.1 syntax.
The application layer uses a series of protocols based on the notion of a resource. A resource defines an operating unit that is available to the applications running on a module. In order for it to be used, each resource must carry a set of objects and a protocol so that they can be mutually exchanged between resources. The communication with the resource is made when a session is created for the resource in question.
In fact, the decoding systems are generally incompatible with the different companies which offer pay services, these companies being competitors. However, it is preferable that the different encoding systems used are transparent for the user, provided that two interface modules or more are connected to the host, with each dedicated to a given decoding system, to allow simultaneous access to several services. The possibility of connecting at least two modules is also attractive for the user who only watches a single source at a given time, because he will not need to physically interchange the modules when changing from one service to another from a rival source.
For more details concerning the specific aspects of the standardization within the European context, refer to the publication “DVB Document AO17” entitled “Common Interface for Conditional Access and Other Digital Broadcasting Decoder Applications”, the entire disclosure of which is herein incorporated by reference.
Until now, the standards and solutions proposed only concerned systems with a single demodulator 12, which is associated with one or more interface modules 6, as explained in the previous paragraphs.
It should be remembered that in compliance with the digital television transmission protocols, the signals are broadcast in “multiplex” form. Each multiplex has complete data (video and audio) from a group of several channels (typically five channels). A single demodulator 18 can process in parallel any number of channels from a multiplex, but only one multiplex at a time.
Several interface modules 6 can therefore be provided to decode several channels from the same multiplex. In fact the channels from the same multiplex are not necessarily encoded in the same way, as they may come from different companies. The different interface modules 6 thus allow the user to use different channels from the same multiplex in parallel.
By way of example, the patent WO-A-OO 77650 offers a connection architecture of several interface modules on a single demodulator to allow the connection of the demodulator with one of the modules to be controlled automatically. More specifically, this document provides means for automatically recognizing interface modules once they have been inserted into their connector, and switching command means to permit the data flow routes between the host and the modules to be interchanged.
One of the major limits of techniques limited to a single modulator appears when several audio/video sources (channels) from different multiplexes need to be exploited simultaneously. Examples of applications which require two or more audio/video sources to be exploited simultaneously include: the image incorporation function, also called PIP (picture in picture), where the television displays a main picture from one channel and at the same time incorporates another image on part of the screen; and devices combining a television and a VCR and/or DVD (“combis”), where the television and the recording device each have at least one demodulator to permit independent operation (in particular to allow one channel to be watched while another is being recorded).
Given the number of channels available and likely to interest a user, quite often different channels to be processed at the same time do not come from the same multiplex.